docs: describe empty values for any source, not only environment variables

Move the paragraph from the environment section to the general note on Sources order, since an empty file is skipped the same way.

Author: abo3losh1

docs/v3/examples/flags/value-sources.md

@@ -62,11 +62,6 @@ func main() {
 If `cli.EnvVars` contains more than one string, the first environment variable that
 resolves is used.
 
-An environment variable that is set but empty only counts as a value for string and
-bool flags, where it means an empty string and `false`. For other flags it is skipped,
-so it does not satisfy `Required`, does not make `IsSet` true, and does not hide a
-later variable in the list.
-
 <!-- {
   "args": ["&#45;&#45;help"],
   "output": "language for the greeting.*LEGACY_COMPAT_LANG.*APP_LANG.*LANG"
@@ -141,6 +136,11 @@ func main() {
 Note that default values are set in the same order as they are defined in the
 `Sources` param. This allows the user to choose order of priority
 
+An empty value from any source, such as an environment variable that is set but empty
+or an empty file, only counts as a value for string and bool flags, where it means an
+empty string and `false`. For any other flag it is skipped, so it does not satisfy
+`Required`, does not make `IsSet` true, and does not hide a later source.
+
 #### Values from alternate input sources (YAML, TOML, and others)
 
 There is a separate package [altsrc](https://github.com/urfave/cli-altsrc) that adds support for getting flag values